The present invention relates to a DRAM memory capacitor having a dielectric which is disposed between two electrodes and contains BaSrTiO3 (BST) and to a method for producing such a DRAM memory capacitor.
As is known, BST is a xe2x80x9chigh-xcex5 materialxe2x80x9d which is regarded as being particularly promising for use as a dielectric for DRAM memory capacitors owing to its maximum possible electrical charge density, which is, for example, approximately ten times as great as that of silicon nitride. An additional advantageous feature of BST in this case is that its scaling with regard to the layer thickness and operating voltage is well below that for silicon nitride.
It is known from the reference by H. Reisinger et al., titled xe2x80x9cDielectric Breakdown, Reliability and Defect Density of (Ba0.7Sr0.3)TiO3(BST)xe2x80x9d, the VLSI Symposium 98, that the maximum density of the electrical charge on BST, and thus its minimum layer thickness for a given operating voltage is not limited by leakage currents but, in fact, by the electrical breakdown voltage and the life to breakdown and thus, in the end, by the reliability of the BST.
When configuring the life expectancy of a dielectric that is subject to electrical loads, the trapping of electrons plays a significant role. For example, it is known from the reference by, W. Hxc3x6nlein et al., titled xe2x80x9cONO Technologyxe2x80x9d, Applied Surface Science 39 (1989), pages 178-191, that electrons may be trapped as permanently as possible in ONO (oxide-nitride-oxide) structures with layer thicknesses of, for example, 5 nm, 8 nm, 5 nm which leads to a reduced current and thus to an increased life and to a lower defect rate as a consequence of a self-healing mechanism of weak points.
In the specialist world, the expression trapping of charge carriers is understood to be the trapping of charge carriers in a potential well or potential trough. The term is also used in this sense here.
The significance of trapping is now explained. Once a voltage has been applied to a BST layer via electrodes, electrons are trapped in this layer, resulting in the current decreasing as a function of time. Thus, owing to the reduced current, the trapped electrons have an advantageous effect on the life. However, if the polarity of the electrical voltage on the electrodes is reversed, the trapped electrons are xe2x80x9cdetrappedxe2x80x9d (or are released from the trapping points), which returns the layer to its original state once again, and increases the current. Continuous reversal of the voltage polarity on the electrodes, as is necessarily the case in a DRAM memory capacitor, thus leads to the life of this capacitor being considerably shortened.
Finally, it is known from the reference by F. Tcheliebon et al., titled xe2x80x9cOn the Microstructure and Optical Properties of Ba0.5Sr0.5TiO3 Filmsxe2x80x9d, Thin Solid Films 299 (1997) 14-17, that the band gap of BST is a function of the barium content.
It is accordingly an object of the invention to provide a DRAM memory capacitor, and a method for its production, that overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type, which has a BST dielectric and is distinguished by a long life.
With the foregoing and other objects in view there is provided, in accordance with the invention, a dynamic random access memory (DRAM) capacitor. The capacitor includes two electrodes and a dielectric containing BaSrTiO3 (BST) disposed between the two electrodes. The dielectric is formed of at least three layers with a middle layer functioning as a potential well for trapping electrons.
In the case of the DRAM memory capacitor of the type mentioned initially, the object is achieved according to the invention in that the dielectric has at least three layers, with the middle layer representing a potential well for trapping electrons.
During operation of the memory capacitor, trapped charge carriers, in particular electrons, are held permanently in the potential well, that is to say in the middle layer, so that they are not released even if the memory capacitor polarity is reversed a number of times. In consequence, the life of the memory capacitor is considerably increased.
The potential well can be formed by the dielectric being in the form of a triple layer. However, n layers (nxe2x89xa74) may also be provided, instead of three layers. It is preferable for a triple layer to be symmetrical in its width/thickness direction. In this case, two outer layers may be composed of BaSrTiO3 and a middle layer may be composed of BaSrTiO3 with a proportion of Ba and/or Ti which is less than that in the outer layers. The potential well is thus produced by the variation of the Ba and/or Ti content of the middle layer with respect to the outer layers.
The triple layer may have a layer thickness of approximately 50 nm, with the individual layers having layer thicknesses of approximately 17 nm. However, in general, layer thicknesses of 5-30 nm are possible.
In the DRAM memory capacitor according to the invention and whose dielectric constant xcex5 (200 less than xcex5 less than 500) is only slightly dependent on Ba/Sr and/or Ti, the thickness equivalent to silicon dioxide is only a few Angstrxc3x6m (xc3x85).
Pt, for example, may be used for the electrodes of the memory capacitor. However, other materials are also possible.
The method according to the invention for producing the DRAM memory capacitor is distinguished in that the dielectric is produced by CVD (deposition from the gas phase) or by sputtering, and in that once each layer has been deposited, a heat treatment in the form of a tempering or annealing step is carried out in oxygen (O2). Thus, for example, Ba/Sr and/or Ti contents of the precursors are changed during CVD, while the target is switched over during sputtering. This allows the desired change in the layer stoichiometry to be produced in a simple manner.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a DRAM memory capacitor, and a method for its production, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.